The present invention relates generally to audio high fidelity cables or transmission lines wherein the wavelength of signals carried on the transmission lines are generally longer than the length of the transmission lines, such as in audio signal and high fidelity sound reproduction applications. The invention has particular application where the range of frequencies is greater than several octaves and therefore wherein spurious oscillations (ringing) and broadband random noise carried on the transmission line can have potentially significant impact on the fidelity of a complex signal carried by the transmission line.
In contrast to long transmission lines where the impedance of the cable is matched to the impedance of the termination loads, it is conventional in short transmission lines to reduce the impedance, and more particularly the resistance, of the cable to a minimum to reduce the electrical resistive loss in the cable. When used in high fidelity audio signal interconnection cabling, as between components in an audio component system, such an approach introduces secondary problems, such as audible enhancement of the "brightness" frequencies (1000 Hz to 2000 Hz range) and can cause distortion in the desired audio frequency signals. The primary causes of these effects appear to be radio-frequency noise and spurious oscillation (ringing) in the interconnect cables.
It is common practice in audio frequency circuitry to provide a twisted pair of signal conductors as a cable between the output of the audio amplifier and the loudspeaker loads. A typical configuration is a twisted pair of conductors in which one of the conductors is used as signal path and the other is used as a signal return path. Designation of the signal path and the signal return may be arbitrary.
Noise which is internally generated in audio equipment can couple into the audio cable, which can cause a spurious oscillation (ringing) of an electromagnetic nature in the cable. The added noise and resultant ringing on the cable can modulate the signal and can couple into the feedback circuit of the output amplifier, causing distortion in the signal. This coupling is aggravated where the length of the two conductive signal leads are substantially equal. Thus the natural length-wise mode of ringing in each element is nearly identical, which can result in common mode noise. This is an undesirable condition.
It appears further that radio frequency sources can induce distortion in audio signals carried over leads of an unshielded cable. The sources may be either internal (in an amplifier for example) or external (in a television set). The effects of radio frequency noise on audio frequency signals appear to be most pronounced at the higher audio frequencies, where distortion has been noted. The presence of oscillations and modal resonances in cables is believed to result in undesired enhancement in the "brightness" frequencies (1000 Hz to 2000 Hz range). These effects are undesirable if accurate reproduction of recorded music and the like is desired.
A solution to some of the problems in faithful transmission of a signal through a cable has been proposed in U.S Pat. No. 4,754,102 issued June 28, 1988, to T.J. Dzurak. Therein a directional transmission cable is disclosed which has a plurality of insulated conductive wires, wherein at one termination all of the collection of wires are connected together and at the opposing termination less than all of the collection of wires are connected together. The unconnected wire or wires run the entire length of the cable.